When drilling a wellbore in the earth, a drilling fluid may be pumped down a drill string and through a drill bit attached to the end of the drill string. The drilling fluid may also flow through a bottom hole assembly (“BH2A”) located in the drill string above the bit. The BHA may house any number of tools or sensors for performing operations while the drill string is in the wellbore. The drilling fluid is generally used for lubrication and cooling of drill bit cutting surfaces while drilling, transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, and displacing the fluid within the well with another fluid. When drilling is completed, the wellbore remains filled with the drilling fluid.
After drilling, casing is often placed in the wellbore to facilitate the production of oil and gas from the formation. The casing is a string of pipes that extends down the wellbore, through which the oil and gas will eventually be extracted.
The region between the casing and the wellbore itself is known as the casing annulus. To fill up the casing annulus and secure the casing in place, the casing is usually “cemented” in the wellbore. Before and even after casing is installed, the well may require wellbore treatment that is referred to as stimulation. Stimulation involves pumping stimulation fluids such as fracturing fluids, acid, cleaning chemicals, and/or proppant laden fluids into the formation to improve wellbore production. The stimulation fluids are pumped through the casing and then into the wellbore. If the casing is installed and more than one zone of interest of the formation is treated, tools may be run into the casing to isolate fluid flow at each zone.
In the case of hydraulic fracturing using fracturing fluids, the fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete, thereby creating a plane of high-permeability sand through which fluids can flow.
Instead of stimulating the formation after installing casing, the well operator may choose to stimulate an uncased portion of a wellbore. To do so, the operator may run a liner extending from the surface into the uncased section of the wellbore with inflatable element packers to isolate the portions of the wellbore. Multiple packers allow the operator to isolate segments of the uncased portion of the wellbore so that each segment may be individually treated to concentrate and control fluid treatment along the wellbore. Generally, the packers are run for a wellbore treatment, but must be moved after each treatment if it is desired to isolate other segments of the well for treatment.
The tubing work string, which conveys the treatment fluid, may include a fracturing or jetting tool for delivering the treatment fluid to the cased or uncased borehole. Alternatively, the tubing string can include ports or openings for the fluid to pass into the wellbore and ultimately to the casing or the formation. Where more concentrated fluid treatment is desired in one position along the wellbore, a small number of larger ports may be used. Where it is desired to distribute treatment fluids over a greater area, a perforated tubing string may be used having a plurality of spaced apart perforations through its wall. The perforations can be distributed along the length of the tube or only at selected segments. The open area of each perforation can be pre-selected to control the volume of fluid passing from the tube during use.
While the introduction of stimulation fluids to the formation can increase formation fluid flow therein and production therefrom, such as by fracturing the formation to create additional fluid flow paths, further fluid flow enhancement will optimize production. Well stimulation deficiencies are overcome by the principles taught herein.